1. Field of the Invention
The present invention concerns a power source for operation of a deflection coil for an electron beam of an x-ray tube. The power source is of the type having a voltage source and a bridge circuit that is connected with each end of the deflection coil via, at each end, one power switch in series connection with opposite poles of the voltage source.
2. Description of the Prior Art
In x-ray devices, x-ray tubes are used to generate x-ray radiation. In the x-ray tube, electrons are accelerated by an electric field, at the x-ray voltage, from a cathode to an anode. Upon striking the anode, the electrons generate characteristic x-ray radiation as a result of their kinetic energy. The direction and form of the generated x-ray beam are determined by the condition and alignment of the surface of the anode as well as by the direction and focal spot contour of the electron beam striking on the anode. In order to generate a directed and intensive x-ray beam in the desired direction, the electron beam is focused and directed at a specific location of the anode surface.
The anode is significantly heated by the kinetic energy of the incident electrons. The electron beam therefore is not statically focused at a point, but rather is oscillated within a specific region in order to enlarge the focal spot on the anode surface and to better distribute the thermal load. The properties of the x-ray beam furthermore can be selectively influenced dependent on the size and contour of the focal spot. In addition to this, there are diagnostic applications In which, simultaneously or in the shortest possible temporal succession, x-ray beams are required from two different directions. Such x-ray beams can be generated by one and the same x-ray tube, by moving the electron beam back and forth in rapid temporal succession between two different focal spots on the anode surface. The movement of the electron beam over the anode surface can be accomplished by a deflection using electromagnetic fields.
Although electrical fields also are used to focus the electron beam, the deflection predominantly occurs using magnetic fields. These are generated by deflection coils that are arranged around the electron beam between cathode and anode, Depending on the requirements for the sharpness of the focusing, the complexity of the focal spot shape, and the desired freedom for deflection of the electron beam, one or more deflection coils can be provided.
The magnetic field generated by the coils is varied by the coil current. The change of the focal spot contour by the movement of the electron beam thus is effected by changing the coil current. The back-and-forth movement of the electron beam between two separate focal spots or different focal spot shapes also is effected by somewhat complex and rapidly ensuing variations of the current in the deflection coils. For this, the coil current must be modified, given changes of the x-ray voltage that accelerates the electrons from the cathode to the anode of the x-ray tube, in order to achieve the retention of the focal spot position; the coil current thus is varied, dependent on the x-ray voltage.
To generate the varying coil current, a power source is necessary that can track the x-ray voltage sufficiently fast enough for modifying the current. The current must be generated in sufficiently precise amounts in order to ensure a stable focal spot position, and it must be exactly variable for generation of the focal spot size an shape. Moreover, tolerances of the x-ray tube or the x-ray voltage must be correctable by a disturbance variable compensation or regulation of the coil current, and a suitable behavior of the power source given failures of the x-ray voltage as a result of tube arcings must be ensured. Not least, the power source should be as small as possible with regard to application in computed tomography (in which it rotates around the examination subject with high rotation speed together with the x-ray tube) and should exhibit a high efficiency to reduce the heat load.
It is known to generate the coil current by means of classical power supply technologies. However, the inductive transformation in power supplies does not allow sufficiently fast modulation of the current. It is additionally known to produce the coil current by means of a function generator and subsequently connected power amplifiers with superimposed current regulation. However, this assembly requires a large structural volume. Moreover, power amplifiers operate with too low an efficiency for the applications described above, Furthermore, the current cannot be regulated with a sufficient stability given large inductive loads (as the deflection coils are) due to their self-induction.
A power source is known from European Application 0 374 289 that is based on the use of power switches. The deflection coil is switched via a bridge circuit of four power switches. To activate the deflection coils, respectively two power switches arranged across from each other are opened, and the deflection coil thus is supplied with a supply voltage. This arrangement enables sufficiently fast switch times in order to ensure a sufficiently fast variation of the coil current, however, the quantity to be varied, the coil current, is not taken into account in the activation of the power switches. A control of the coil current is thereby provided. A control offers no protection against malfunctions as a result of induction-dependent overswings or other interfering influences. Moreover, the circuit does not suitably react given the occurrence of a failure of the x-ray voltage as a result of tube arcings.